Carbon sequestration in woody biomass of mulga (Acacia aneura) woodlands: confidence in prediction using the carbon accounting model FullCAM

Decreased grazing and/or cessation of land clearing across Australia’s rangelands are being used to promote carbon sequestration through regeneration of woody biomass, predominately in Acacia aneura (mulga) woodlands. Changes in carbon stock are predicted using the carbon accounting model FullCAM. We collated datasets to assess the level of confidence in applying FullCAM to mulga regeneration across south-western Queensland and north-western New South Wales, with respect to model accuracy, specificity, and comprehensiveness. We found that FullCAM predictions were moderately accurate, with independent verification sites (N = 102) indicating model efficiencies of 48–70% and bias of −3.50 to −0.99 Mg DM ha−1, depending on calculation method. To ensure accuracy and to reduce risks of over-prediction, it is recommended FullCAM should be limited to sites with regeneration ages of ≤25 years and with levels of pre-existing above-ground biomass less than approximately 5 Mg DM ha−1. The paucity of data from mulga ecosystems in central and western Australia was identified as an important research gap. Regarding specificity, FullCAM has been calibrated to average rates of regeneration, generalised across a range of vegetation types, disturbance histories, and grazing management practices. This generalisation ensures accuracy when applied over broad spatial domains, but may limit the model’s accuracy at specific locations. For example, at the site scale, long-term grazing exclosure experiments (N = 34) have shown a wide range of regeneration outcomes (−0.52 to 1.85 Mg DM ha−1 year−1, with an average of 0.29 Mg DM ha−1 year−1), with site-scale contributors to this variability including the proportion of mulga in the total biomass, and the degree of change in grazing intensity (e.g. exclusion of livestock only, cf. exclusion of livestock plus native and feral animals). Regarding model comprehensiveness, new field data suggest that FullCAM could be extended to include standing dead pools of woody biomass, which contribute, on average, 17% of total woody biomass in mulga woodlands.